Metallurgical process



Feb. 19, 1963 G. NAESER 3,078,158

METALLURGICAL PROCESS Filed Aug. 7, 1961 CARBON CONTENT v. HEATING TIMEHEATING TIME (HRS) 2 a. a d O O o womuamoa Noeavo INVENTOR. GerhardNaeser BY h/s af/omeys United States Patent 3,078,158 METALLURGHCALPRUCESS Gerhard Naeser, Dnisburg-Huckingen, Germany, assignor toMannesmann Aktiengesellschaft, Dusseldorf, Germany, a German companyFiled Aug. 7, 1961, Ser. No. 129,707 3 Claims. (Ci. 75--.5)

This invention relates to a process for decarbur-izing ferroalloypowders, especially powdered ferrochromium.

Various elements used in alloying steel, such for example as chromium,manganese, silicon, tungsten, molybdenum and vanadium are normally addedto the steel as so-called ferroalloys, i.e. alloys of iron withrelatively large proportions of the particular metal to be added. Thus,for example, ferrochromium usually contains 60-75% chromium, the balancebeing chiefly iron with up to 3% Si.

It is well recognized that these ferroalloys must have as low a carboncontent as possible. However, when the alloys are made according toconventional techniques, they have a high carbon content, e.g. up tocarbon, and must therefore be decarburized. Several processes havehitherto been proposed for carrying out this decarbunization.

In one prior process, finely ground ferrochromium powder is heated underoxidizing conditions so that most of the carbon is removed as carbonmonoxide. However, in this process oxidation of the metal occurssimultaneously with oxidation of the carbon and it is thereforenecessary subsequently to subject the partially oxidized alloy toheating in a reducing atmosphere. It is not generally possible to reducethe carbon content of alloys much below 1% by this process.

In another conventional process a finely powdered ferrochromium rich incarbon is partially oxidized to obtain a ratio of oxygen to carbon above1.3 and is then heated at 1300 C. under reduced pressure for an extendedtime, i.e. about hours. The reaction between the carbon and oxygenresults in an alloy containing only about 0.02% carbon.

It has also been suggested to mix powdered ferrochromium rich in carbonwith finely powdered chromium oxide or iron oxide in proportions suchthat the amount of oxidizing agent is stoichiometrically adjusted to thecarbon content. The subsequent extended heating in vacuo at about 1300C. results in ferrochromium containing about 0.01% carbon.

Finally, it is known to atomize molten ferrochrome by means ofcompressed air, steam or other oxidizing gases in such a way that theresultant ferrochrome powder is partially oxidized and the degree ofoxidation and carbon content of the powder are such that the oxygencontent is sufiicient to convert the larger part of the carbon, uponsubsequent heating under reduced pressure, to carbon monoxide.

It will be appreciated that while the several processes described abovewhich involve heating under reduced pressure can give quite low carboncontents, the time involved to achieve this is objectionably long.

It has now been found that still lower carbon contents can be obtainedwith much shorter heating times if to the ferro-alloy powder, beforeheating, there is added a small amount of an alkali compound.

The invention therefore comprises, in a process for decarburizingpowdered ferroalloys having high carbon contents by heating or roastingunder sub-atmospheric pressure, the improvement of adding an alkalicompound to the powder before heating.

The drawing is a graph showing carbon content as a function of heatingtime for two alloy powders under- 3,078,158 Patented Feb. 19, 1963 "icegoing decarburization. One of the powders has an alkali compound addedin accordance with the invention, while the other does not.

Any ferroalloy can be treated according to the inven tion. Preferably,however, the powder is one which has been made by atomizing a melt ofthe alloy with compressed air, steam or other oxidizing gas and whichcontains an amount of oxygen equivalent to its carbon content. Theinvention is especially suitable for decarburizing ferrochromium.

The alkali compound added may be any suitable compound of the alkalimetals. Potassium fluoride and potassium carbonate have been foundespecially useful. The amounts added are very low, normally not morethan about 5%. Thus, for example, for potassium fluoride an addition of0.05 to 5%, preferably 0.1 to 0.3% by weight of the alloy powder isprescribed.

The technique used to add the alkali compound may vary. Thus, forexample, it may itself be reduced to a finely divided powder andphysically mixed with the alloy powder; or a solution of the alkalicompound may be used to impregnate the alloy powder.

The invention will be further described with reference to the followingspecific example which is given for purposes of illustration only and isnot to be taken as in any way limiting the invention beyond the scope ofthe appended claims.

Example A ferrochrome melt containing, by weight, 70% Cr, 1% Si, and8.5% C, the balance being iron, was atomized with compressed air to givea powder in which the weight ratio of carbon to oxygen was about 1:13.This powder was then impregnated with a potassium fluoride solution sothat the total amount of potassium fluoride deposited on the powderamounted to 0.2% by weight of the powder. The powder was then roasted at1300 C. at a reduced pressure of between 0.01 and 30 mm. Hg vacuum. Thecourse of the decarbur-ization is shown graphically in the drawing.Referring to curve B, the carbon content after only one hour had alreadyfallen to 0.014%. After three hours it was 0.004% and after 10 hours thecarbon content could no longer be determined analytically.

In contrast, curve A, shows the course of decarburization of the samepowder without alkali addition at the same temperature and pressure. Aswill appear, in this operation it was impossible, even after 20 hours ofheating, to attain the extremely low carbon content obtained in theprior run.

I claim:

1. A method for the decarburization of ferrochrome powder which has beenreduced to powdered form by atomization with air and contains equivalentamounts of carbon and oxygen, which comprises impregnating the powderwith potassium fluoride and then roasting the impregnated powder underreduced pressure to achieve substantially complete decarburization.

2. The process claimed in claim 1 wherein the potassium fluoride is usedin a proportion between about 0.05% and about 5% by weight of the alloypowder.

3. The process claimed in claim 1 wherein the potassium fluoride is usedin a proportion of between about 0.1 and about 0.3% by weight of thealloy powder.

References Cited in the file of this patent UNITED STATES PATENTS

1. A METHOD FOR THE DECARBURIZATION OF FERROCHROME POWDER WHICH HAS BEENREDUCED TO POWDERED FORM BY ATOMIZATION WITH AIR AND CONTAINS EQUIVALENTAMOUNTS OF CARBON AND OXYGEN, WHICH COMPRISES IMPREGNATING THE POWDERWITH POTASSIUM FLUORIDE AND THEN ROASTING THE IMPREGNATED POWDER UNDERREDUCED PRESSURE TO ACHIEVE SUBSTANTIALLY COMPLETE DECARBURIZATION.